Map

A map is a collection that has keys, and values for each key. And the term may refer to a function that applies a method to each element in a collection.

In VB.NET, the term "map" does not have a specific meaning, so we must use the Dictionary type and Select function. A lambda expression can be used as an argument to Select.

`Collection` example

How can we use a map collection in VB.NET? First we must create a new instance of the Dictionary, with its generic type syntax.

Part 1 We add 2 keys, with associated values, to the Dictionary after creating it.

Part 2 When using a Dictionary, we can loop over its keys and values with a For-Each loop.

Part 3 It is possible to access the map at a key, but this will throw an exception if the key is not found.

Part 4 With TryGetValue, we can access a value at a key more safely—if the key is not found, no exception is thrown (False is returned instead).

Module Module1

    Sub Main()

        ' Part 1: create new Dictionary and add 2 keys and values.
        Dim map = New Dictionary(Of String, String)()
        map.Add("cat", "orange")
        map.Add("dog", "brown")

        ' Part 2: loop over the map.
        For Each pair In map
            Dim key = pair.Key
            Dim value = pair.Value
            Console.WriteLine($"{key} / {value}")
        Next

        ' Part 3: access value at key.
        Dim result = map("cat")
        Console.WriteLine(result)

        ' Part 4: access value at key with TryGetValue (safer).
        Dim result2 = Nothing
        If map.TryGetValue("dog", result2)
            Console.WriteLine(result2)
        End If

    End Sub

End Modulecat / orange
dog / brown
orange
brown

Modify array

Another use of the term "map" means to apply a function each element of a collection (like an array). This can be done with the Select function in VB.NET.

Tip We pass a lambda expression with the Function keyword as an argument to the Select Function.

Tip 2 In languages like Python, the "map" function is equivalent to Select() in VB.NET.

Module Module1

    Sub Main()

        Dim array As String() = { "bird", "fish" }

        ' Print original array.
        Console.WriteLine(String.Join(",", array))

        ' Call a Function on each element in the array, modifying the element.
        Dim result = array.Select(Function (x)
            Return x.ToUpper()
        End Function).ToArray()

        ' Print result.
        Console.WriteLine(String.Join(",", result))

    End Sub

End Modulebird,fish
BIRD,FISH

VB.NET programs can create map collections with a Dictionary, and map elements in collections with the Select function. Mapping things is an essential task in this language.

Map

Collection example

Modify array

`Collection` example